One such part-time four-wheel transfer case is disclosed in, for example, Japanese Utility Model Publication No. 61-9791 entitled "2-4 Drive Mode Shift Mechanism for a Four Wheel Drive Vehicle", published Mar. 28, 1986. The transfer case taught by the above mentioned Publication has a hydraulic actuator for actuating means to shift a transfer case into two-wheel or a four-wheel drive mode of operation and solenoid valves through which the hydraulic actuator is controlled. The solenoid valves are electrically controlled in such a way to control the hydraulic actuator to shift the transfer case into the two-wheel drive mode of operation if the vehicle is traveling at a high speed. On the other hand, if the vehicle is traveling at a low speed, the transfer case is shifted into the four-wheel drive mode of operation when the transfer case is in a low speed drive range and is selectively shifted into a desired drive mode between the two-wheel and four-wheel drive modes of operations according to a condition of a drive mode shift switch operated by the driver when the transfer case is in a high speed drive range.
In part-time four-wheel vehicles equipped with transfer cases having a center-differential shiftable between locked, and unlocked or free drive modes of operations, the center-differential is allowed to be selectively shifted from one drive mode of operation to another among three drive modes of operations, namely, two-wheel, four-wheel differential-locked and four-wheel differential-free drive modes of operations. In this regard, the provision of electric shift means is contributive to an easy and quick shift operation.
A problem associated with such apparatus is that it is hardly possible to maintain a high travelling stability of the vehicle because, when the vehicle is shifted from the four-wheel drive mode of operation to the two-wheel drive mode of operation, disconnected or non-driving wheels lose their tractive force.